Analog switch is a type of interface for passing or blocking analog signal between analog circuit and digital control circuit. Along with the development of integrated circuits (IC) in the past few years, the technique in analog switch is improved a lot, in that it can work under very low voltage, have very low conduction resistance and very small packaging size. Due to these improvements, analog switches are widely used in sensing apparatuses, communication products and multi-media systems, etc.
A standard complementary metal oxide semiconductor (CMOS) analog switch comprises an n-channel MOS (NMOS) and a p-channel MOS (PMOS) that is coupled in parallel with the NMOS. The CMOS analog switch allows signals transfer bi-directionally. However, this parallel configuration leads that the conduction resistance changes along with the voltage of the input signal. At the meantime, the power supply voltage affects the conduction resistance a lot, and the conduction resistance will remarkably increase when the supply voltage decreases. In one example, when the power supply voltage is 5 volts, the conduction resistance of a conventional CMOS analog switch is about 8 ohms, and when the supply voltage is set in 12 volts, the conduction resistance of the CMOS analog switch decreases to about 3 ohms.
Conduction resistance for conventional CMOS analog switch in rated input voltage range and rated power supply voltage range changes a lot, thus the flatness of conduction resistance is poor. Accordingly, it becomes a critical problem for analog switch on keeping the flatness of conduction resistance with changing input voltage and power supply voltage.